An experiment was carried out to determine the Kd (equilibrium dissociation constant) values of two newly developed ligands at the same receptor target. Drug C: Kd = 0.8μM Drug D: Kd = 8nM Based on this information, it could be concluded that at this receptor, Drug C:

Question

BlackTom AI · Accepted Answer

Kd (equilibrium dissociation constant) is inversely related to receptor affinity: a lower Kd means the ligand binds more tightly (higher affinity). Kd is the concentration of ligand at which half of the receptors are occupied, so comparing Kd values directly compares affinities.

Step-by-step reasoning and calculations:
1. Convert both Kd values to the same units (nanomolar, nM) for direct comparison.
   - Drug C: 0.8 μM = 0.8 × 10^–6 M = 800 × 10^–9 M = 800 nM.
   - Drug D: 8 nM (already in nM).
2. Compute the ratio of Drug C Kd to Drug D Kd:
   - Ratio = 800 nM / 8 nM = 100.
   This means Drug C’s Kd is 100 times larger than Drug D’s Kd.
3. Interpret the ratio: because Drug C has a Kd 100-fold higher, it requires a 100-fold greater concentration to occupy 50% of receptors compared with Drug D. Therefore Drug C binds the receptor less tightly (lower affinity).

Why the correct answer is correct:
- Kd directly measures binding affinity at equilibrium. Drug D’s much lower Kd (8 nM) indicates it has much higher affinity for the receptor than Drug C (800 nM). Thus the statement “Drug C has lower affinity than Drug D” is the correct conclusion.

Why the other options are incorrect:
- “Is a more potent agonist than Drug D.” Potency depends on both affinity and intrinsic efficacy; Kd alone does not indicate agonist/antagonist identity or efficacy, so you cannot conclude greater potency or agonism from Kd.
- “Has higher affinity than Drug D.” This contradicts the numeric comparison; Drug C’s Kd is larger, so its affinity is lower.
- “Is a less potent antagonist than Drug D.” Antagonist potency likewise depends on functional assays and mechanism; Kd alone does not establish antagonist potency or even whether the ligand is an antagonist.

Conclusion: Drug C has lower affinity than Drug D, matching the provided correct answer.

An experiment was carried out to determine the Kd (equilibrium dissociation constant) values of two newly developed ligands at the same receptor target. Drug C: Kd = 0.8μM Drug D: Kd = 8nM Based on this information, it could be concluded that at this receptor, Drug C:单项选择题

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